High speed steel hardening bath



Patented Oct. 20, 1942 HIGH SPEED STEEL HARDENING BATH Haig Solakian andLouis Sarvis, NewHaven, Conn.

No Drawing. Application April 29, 1939,

Serial No. 270,915

1 Claim.

The present invention relates to a fused salt bath especiallyconstituted for the treatment of high speed steels.

This is in part a continuation of our prior application Serial No.120,933, filed January 16, 1937.

The materials comprising the bath are combined in such manner as toconstitute a composition which has a melting point at a relatively lowtemperature such as 700 to 720 F.

An important feature of the present invention is that in view of the lowmelting point of the bath and the complete fluidity at temperaturesaround 900 F., the salts readily drain off the articles treated and theslight residue when they are removed from the bath is readily removed inboiling water due to the combined action of dissolving and mechanicaldisintegration so that the hardened parts are easily cleaned after thehardening operation.

A further feature of the bath is that the bath may be replenished andrenewed by additions of the combined materials in order to replace thematerial which is removed from the bath mechanically due to the stickingof the materials to the surface of the articles taken from the bath.

The high speed steels for which this bath is especially constitutedabsorb both carbon and nitrogen from the bath and, therefore, the actionof this bath is known in the art as nitriding.

Another advantage of the present composition is that, since it may beused at a relatively low temperature it will produce a balanced type ofnitriding and there is little tendency of distortion of work. This isimportant where work such as tools, dies, etc., must be maintainedrelatively close to predetermined dimensions.

The preferred constituents of the composition are: barium chloride, orany other alkaline earth metal chloride; sodium cyanide, or any otheralkali metal cyanide; sodium carbonate, or any.

other alkali metal carbonate; potassium carbonate, or potassium chlorideor any other alkali metal chloride; calcium fluoride, or any otheralkaline earth metal fluoride, or sodium fluoride, or any other alkalimetal fluorides, or sodium chloride.

The preferred formula comprising the following ingredients insubstantially the following tages as are given above.

proportions and wherein the preferred percentages are given by weight:

Per cent Barium chloride 12 Sodium cyanide 33 Sodium carbonate 43Potassium carbonate 8 Calcium fluoride 4 It is, to be understood thatth'ese proportions are only the preferred proportions and that the .bathmay be operated by the materials in the proportions having substantiallythe following limits:

Per cent Barium chloride -1 5 to 25 Sodium cyanide 10 to 45 Sodiumcarbonate 15 to 45 Potassium carbonate 5 to 20 Calcium fluoride 2 to 10Sodium fluoride may be substituted for calcium fluoride within the samelimits of percen- The barium chloride, sodium cyanide, sodium carbonateand potassium carbonate are thoroughly fused together and then thecalcium fluoride or sodium fluoride is added to the top of the bath andthe fusing is continued until the materials are thoroughly combined. Themass is then allowed to cool and is broken up into small pieces suitablefor packaging and shipping. These broken pieces are adaptable for use bybeing melted in the nitriding pot, and the bath is maintained at thedesired volume by adding the broken pieces thereto from time to time.

.With the materials in substantially the above preferred proportions,the melting'point of the composition is approximately from 700 F. to

720 F. The advantage of this low melting point is that the bath is veryliquid at temperatures of 900 F. which makes it particularly suitablefor producing secondary hardness on high speed steel tools attemperatures from 900 F. to 1150 F. The work may be heated in this bathfor a time period ranging from five minutes to several hours, bothtemperature and time being dependent upon the size of the piece of Workand its particular use.

This bath as has been previously stated is more especially designed foruse with high speed steels Per cent Tungsten 17.5 to 18.5 Chromium 3.75to 4.25 Vanadium 1.0 to 4.0 Carbon 0.60 to 0.85

Another type of high speed steel known the art as molybdenum type Acomprises an alloy of iron with I Per cent Chromium 3.75 to 4.25Tungsten 2.0 to 3.0 Vanadium 1.0 to 4.0 Molybdenum 6.0 to 8.0 Carbon0.65 to 0.85

Another type of molybdenum liigh speed steel is still known in theart-as molybdenum type B in which iron is alloyed with Per centMolybdenum 8.0 to 10.0 Chromium 3.75 to 4.25 Vanadium 1.0 to 4.0 Carbon3.75 to 4.25

A still further type of high speed steel is known as the cobalt type andcomprises iron alloyed with Per cent Tungsten 12.0 to 18.0 Chromium 3.75to 4.25 Vanadium 1.0 to 2.0 Cobalt 4.0 to12.0 Carbon 0.65 to 0.85

The high speed steels referred to prior to treatment in this salt bathhave a hardness which usually is between 63 to 65 on the C-scale,Rockwell. After high speed steel has been treated fifteen to thirtyminutes in this salt bath at a temperature of approximately 1050" F.,the surface hardness increases to about 75 C-scale, Rockwell as measuredby the superficial Rockwell method and converted to the standardRockwell reading on'the C-scale. This surface hardness is substantialand closely approaches the hardness of a diamond.

A polished surface of high speed steel treated as above specified whenviewed under a microscope having one thousand diameter magnificationappears as though there had been some segregation of carbides on thesurface. Apparently there has also been an absorption of nitrogen andthe formation of nitrides as well .as carbides. Tools and dies treatedas above specified with this surface hardening bath resist wear and havea greatly increased cutting efficiency. The time and temperaturereferred to depends upon the use of the tools. The preferred depth ofpenetration of the high speed steel is extremely shallow in themagnitude of .0001 or .0002 of an inch. However, its effect on thecutting efficiency of the tool is out of proportion to the depth as itincreases the efliciency of high speed steel tools from two to tentimes.

During the operation of the bath, a black crust forms on the top of thebath which acts as a blanket and aids in reducing the oxidation effectby the atmosphere on the bath. This 'fidllces substantially the rate ofsodium cyanide decomposition.

Another important effect of the crust on top of the bath is that itincreases the vapor pressure within the bath and this in turn increasesthe effective operation of the bath and causes a deeper penetration ofthe case in a lesser time than would be possible if there were no cruston the bath. The main reason for the crust, however, and its mostdesirable characteristic, is to preserve the sodium cyanide content andits interaction with the other materials. The bath in accordance withthe present invention does not form a sludge as do ordinary cyanidebaths? The barium chloride in the bath appears to act as a carrier ofthe cyanogen radical and contributes to the reducing of the meltingpoint of the bath. Preferably, this particular chemical is used in smallproportions in combination with the other materials to reduce the amountof the insolubles as its interaction with the sodium carbonate tends toproduce barium carbonate and sodium chloride as salts. The bariumcarbonate is not easily soluble in water but when combined with theother chemicals of the present bath, the residue on the articles treatedis easily removed in a water wash.

The sodium carbonate is preferably used in a relatively high percentageof the composition in that its chemical reaction with the otherconstituents reduces the melting point of the bath and it has been foundin actual use that the relation of the sodium carbonate in itsproportion to the other materials operates best in accordance with theproportions in the preferred formulae given above.

Potassium carbonate also contributes materially to the fluidity of thebath and thus aids in reducing the melting point as well as in aidingthe washing of th materials taken from the bath in that it is readilysoluble in hot water. The effect of the potassium carbonate is notclearly understood, but it apparently has an affinity for and aninteraction with the calcium or sodium fluoride when the fluoride isintroduced into the bath.

The sodium cyanide is the main source of the carbon and nitrogen whichis consumed during the treatment of high speed steel articles in thebath. The amounts of sodium cyanide in the formulae are based on theassumption that a consistent, uniform loss from the bath will occurrelatively to a given area of high speed steel treated in the bath, andits proportional amount with the other chemicals will produce suificientCO gases to maintain its equilibrium for treatment of high speed steelarticles at the desired temperatures.

The calcium or sodium fluoride appears to be effective as a catalyst inthe bath and reacts with the other chemicals. thereby acting ingenerating free carbon whichitends to form the black film or crust onthe top surface of the bath as heretofore specified. The stability ofthe bath and its slow rate of decomposition appear to result directlyfrom the calcium fluoride and in that the factor of the fluoridecomposition in the bath is held at a low point which preventsaccelerated decomposition.

Potassium chloride may be substituted for potassium carbonate; but therate of decomposition is somewhat increased and the melting point of thebath is raised to a slight degree where this substitution is made.

Calcium chloride may be substituted for calcium or sodium fluoride.While calcium chloride tends to act as a catalyst, it is not as stable amaterial as'calcium or sodium fluoride and it tends to produce morewashing insolubles apparently as a result of its interaction withpotassium carbonate and the iron pot in which the bath is melted. Thecalcium chloride has a tendency to attack the iron 'or steel pot formingan insoluble compound of calcium and iron and thus forms as.

crust which adheres to the side of the pot or forms a sludge whichsettles to the bottom of the pot. In either case, it-causes localover-heating carbonate. This causes a somewhat greater re- \bspedf steelarticles comprising the fusion of the action between the cyanide and thebase chemicals than would otherwise occur if the calcium chloride couldbe added at the surface of the bath after .the other ingredients arefused as is 5 the case with the calcium or sodium fluoride.

The calcium chloride also tends to promote a rust condition which is notdesirable, on high steel tools treated in the bath. I

"-13 what wegclaim is:

salt bath for surface hardening high following ingredients insubstantially the following percentages by weight: barium chlorid 12%;

sodium ycyanide 33%; sodium carbonate 43%;

15 potassiunrcarbonate' 8%; and calcium fluoride 4%. 1 HAIG SOLAKIAN.

LOUIS SARVIS.

